Automatic transmissions, especially for motor vehicles encompassing planetary gearsets with the aid of frictionally based shifting elements, such as clutches and brakes, can be engaged and the transmission normally possesses a slip-action startup element and is optionally furnished with a bypass clutch. The bypass clutch would be supplied in the form of a hydrodynamic torque converter or a hydrodynamic clutch.
A multi-speed transmission is disclosed by DE 102 13 820 A1. This cited transmission includes one input shaft and one output shaft, which are mutually co-axially aligned. Further total of three planetary gearsets and six frictional-based shifting elements are included. For the transfer of the speed of rotation of the input shaft to the output shaft, the cited transmission provides two drive paths. Two of the planetary gearsets form a shiftable main gearset, which is designed as a two-carrier, four shaft transmission and is further known as a Ravigneaux planetary gearset or, alternately as a so-called Simpson planetary gearset. The output element of the main gearset is bound with the output shaft of the transmission. The remaining one of the three planetary gearsets serves as a simple planetary gearset and forms a non-shiftable, transfer gearset, which is rigidly bound to the input shaft and generates a speed of rotation on its output side which, besides the speed of rotation of the input shaft, can be transmitted to various input elements of the main gearset. By selective engagement of two respective six frictional shifting elements, which are namely clutches and brakes, a total of eight forward gear ratios can be engaged in a manner free of range shifting. In this way, the shifting action is such that in the case of a change from one gear ratio into the next respective successive gear ratio, whether higher or lower, then only one of the previously engaged shifting elements need be disengaged and a previously disengaged shifting element is engaged.
In U.S. Pat. No. 5,429,557, an axially aligned, comparatively compactly assembled, multi-speed automatic transmission of planetary gearset design, which has a transfer gearset and a principal double planetary gearset constructed of two planetary gearsets, which are coupled together as is done in a two-carrier, four shaft planetary transmission. In an embodiment of this transmission, which serves well for installation in a vehicle having a front-end, transversely directed drive, the input shaft and the output shaft are arranged to be axis-parallel to one another. Starting with this arrangement, the transfer gearset is constructed in the transfer method, with two spur gear ratios of a constant ratio having overdrive characteristics and the two planetary gearsets of the main gearset are radially nested, one within the other, wherein the axes thereof run parallel to the axis of the input shaft of the likewise transverse transmission. In the use of a total of five frictionally based shifting elements in this way, only six forward, group shift-free stages are operationally available for the patent.
Furthermore, DE 199 49 507 A1 of the Applicant discloses a multi-speed transmission, wherein two non-shiftable, transfer planetary gearsets are placed on the input shaft, which generate two speeds of rotation which, besides the speed of rotation of the input shaft, in addition, can also be transmitted to various input elements of a multi-speed main planetary gearset by way of selective engagement of shifting elements, acting upon the output shaft AB with the result that, for selective changing from one gear ratio to another, be that a higher or lower gear ratio, then of the two immediately engaged shifting elements, respectively, only one need be engaged or disengaged. The principal planetary gearset as before, is once again constructed in the manner of a double planetary gearset this being a two-carrier, four shaft planetary gearset, wherein two double planetary gearsets, by way of two elements, are rigidly coupled to one another. When five shifting elements are put into use, then seven forward gear ratios become range-free shiftable. In the case of using six shifting elements, even nine or ten forward gear ratios become available. All four planetary gearsets are, in this embodiment, co-axially aligned to each other and to the axially disposed input shaft.
Within the framework of DE 101 15 983 A1 of the Applicant, a multi-speed transmission is described, which possesses one drive shaft which is connected with a transfer gearset, and has one output shaft which, in turn, is bound to an output-sided gearset assembly. A maximum of seven shifting elements are further supplied, the selective shifting of which, enables shifting into at least eight forward gear ratios (without range shifting). The transfer gearset assembly consists of a shiftable, or a non-shiftable planetary gearset, or, alternately, of a pair of non shiftable, mutually coupled planetary gearsets. The output-sided planetary gear assembly is assembled, as in the case, of a two-carrier, four shaft transmission with two shiftable output-sided planetary gearsets and possesses four free shafts. The first free shaft of this two-carrier, four shaft transmission is connected with the first shifting element, the second free shaft is bound to the second and third shifting element, the third free shaft is attached to the fourth and fifth shifting element and the fourth free shaft is connected to the output shaft. In the case of a multi-speed transmission with a total of six shifting elements, the proposal is to additionally connect the third free shaft or the first free shaft of the output-sided gear group additionally to the sixth shifting element. For a multi-speed transmission having a total of seven shifting elements, it is proposed to connect the third free shaft additionally with the sixth shifting element and the first free shaft additionally to a seventh shifting element. In these cases, all planetary gearsets of the transmission are arranged to be mutually co-axial.
In the Applicant's DE 101 15 995 A1, a plurality of other multi-speed transmission are disclosed, wherein four shiftable mutually coupled and co-axially arranged planetary gearsets and six or seven frictionally-based shifting elements are provided, the selective shifting of which transfer a speed of rotation of an input shaft of the transmission in such a manner to an output shaft of the transmission that nine or eleven forward gear ratios and at least one reverse gear ratio become available as shifting positions. In accord with a shift-logic diagram of the transmission, it may be seen that in each gear ratio two or three shifting elements are engaged while, upon a change from one established gear ratio position to the next successive higher or lower gear ratio, for the avoidance of range shifting, respectively only one engaged shifting element need be disengaged and one previously disengaged element need be engaged.
The present invention has the purpose of proposing a multi-speed transmission, which possesses at least eight forward gear ratios and at least one reverse gear ratio wherein, under the usage of a total of four planetary gearsets, the least possible number of shifting elements need be included. In this arrangement, all forward gear ratios, when in a sequential gear change, can be so shifted without range shifting, in other words, in a case of change from one forward gear ratio to the successively next higher or lower forward gear ratio, respectively, only one previously engaged shifting element need be disengaged and a previously disengaged shifting element need be engaged. In addition, the transmission is expected to exhibit a large ratio spread, where comparative harmonic gear separations are concerned and have a favorable degree of efficiency in the main gear ratios used mostly in driving. Accordingly, a comparatively small amount of slippage and tooth wear is incurred. The transmission should be adaptable, in particular, for usage with non co-axial input and output shafts and correspondingly, the axial length of the transmission should be held to the shortest possible dimensioning.